Citrus Diseases

Anthracnose

Scientific name

Disease cycle

Anthracnose is a primary colonizer of injured and senescent tissue. The organism grows on dead wood in the canopy, and it spreads short distances by rain splash, heavy dew, and overhead irrigation. Such movement deposits the spores on susceptible tissues of young leaves or immature fruit. Sexual spores, although less numerous, are significant for long distance dispersal because of their ability to become airborne. Once the spores germinate, they form a resting structure that allows them to remain dormant until an injury occurs or until degreening. The disease is especially troublesome on fruit that are harvested early and degreened for over 24 hours because ethylene stimulates the growth of the fungus.

Symptoms

Leaf - common symptoms are a more or less circular, flat area, light tan in color with a prominent purple margin that at a later phase of infection will show the fruiting bodies of the fungus (tiny dispersed black flecks). Tissues injured by various environmental factors (such as mesophyll collapse or heavy infestations of spider mites) are more susceptible to anthracnose colonization.

Fruit - anthracnose usually only occurs on fruit that have been injured by other agents, such as sunburn, chemical burn, pest damage, bruising, or extended storage periods. The lesions are brown to black spots of 1.5 mm or greater diameter. The decay is usually firm and dry but if deep enough can soften the fruit. If kept under humid conditions, the spore masses are pink to salmon, but if kept dry, the spores appear brown to black. On ethylene degreened fruit, lesions are flat and silver in color with a leathery texture. On degreened fruit, much of the rind is affected. The lesions will eventually become brown to grey black leading to soft rot.

It should be noted that leaves and fruit infected with other diseases (alternaria, citrus canker) may also be colonized by the fruiting bodies of C. gloeosporioides. The fruiting bodies (black flecks) can be seen over the disease of concern.